📚 365 Days: The Science Behind the Year | 365天：一年背后的科学
Every calendar we use is built on a single number: 365. But where does this number come from, and what does it mean in science?
我们使用的每一本日历都建立在一个数字上：365。但这个数字从何而来，在科学上又意味着什么？
1. The Orbit of the Earth | 地球的公转
The Earth moves around the Sun in an almost circular orbit. This journey takes about 365.25 days. That period is called one year.
地球几乎以圆形轨道绕太阳运动。这段旅程大约需要365.25天，这个周期被称为一年。
The orbit is not a perfect circle but an ellipse, as described by Kepler’s laws. The Sun is located at one focus of the ellipse. However, the eccentricity is so small that for most IGCSE calculations we can treat it as circular.
轨道并不是完美的圆形，而是椭圆，开普勒定律描述了这一点。太阳位于椭圆的一个焦点上。但偏心率非常小，因此在大多数IGCSE计算中，我们可以将它近似为圆形。
1 year ≈ 365.25 days
2. One Year Explained | 一年的定义
There are two important ways to define a year: the tropical year and the sidereal year. The tropical year is the time between two successive vernal equinoxes. It is about 365.2422 days and is used in our calendar system.
定义一年有重要两种方式：回归年和恒星年。回归年是从一个春分点到下一个春分点的时间，约为365.2422天，我们的历法采用的就是回归年。
A sidereal year is the time for the Earth to complete one full orbit relative to the distant stars. It is about 365.2564 days. The difference is caused by the slow precession of Earth’s axis.
恒星年是地球相对于遥远恒星完成一次完整公转的时间，约为365.2564天。两者之间的差异由地球地轴的缓慢进动引起。
Tropical year ≈ 365.2422 days
Sidereal year ≈ 365.2564 days
3. The Sidereal vs Solar Day | 恒星日与太阳日
One year contains about 365.24 solar days, but relative to the stars, the Earth actually rotates 366.24 times in that period.
一年包含约365.24个太阳日，但相对于恒星而言，地球在这段时间实际上自转了366.24次。
A solar day is the time between successive moments when the Sun is at its highest point in the sky. A sidereal day is the time for one rotation relative to the stars. The solar day is about 4 minutes longer than the sidereal day.
太阳日是太阳连续两次经过天空最高点的时间。恒星日是相对恒星自转一圈的时间。太阳日比恒星日长约4分钟。
This extra rotation per year explains why there are roughly 365 sunlight days but 366 star-based rotations in a complete orbit.
每年多出的这一点自转，解释了为什么在完整公转中大约有365个阳光日，却有366次基于恒星的自转。
4. Leap Years | 闰年
If the real year is 365.2422 days, a calendar with exactly 365 days would drift against the seasons. To fix this, we add an extra day every four years: February 29.
如果真实一年是365.2422天，那么只有365天的日历会与季节逐渐错位。为了解决这个问题，我们每四年增加一天：2月29日。
The simple rule is: a year divisible by 4 is a leap year. However, century years are not leap years unless they are divisible by 400.
简单的规则是：能被4整除的年份是闰年。但是，整百年份不能被400整除时不是闰年。
- 2016 is a leap year because 2016 ÷ 4 = 504.
- 1900 is not a leap year because 1900 ÷ 100 = 19, but 1900 ÷ 400 is not exact.
- 2000 is a leap year because 2000 ÷ 400 = 5.
- 2016年是闰年，因为2016 ÷ 4 = 504。
- 1900年不是闰年，因为1900 ÷ 100 = 19，但1900 ÷ 400不能整除。
- 2000年是闰年，因为2000 ÷ 400 = 5。
Average year length = 365 + 1/4 − 1/100 + 1/400 = 365.2425 days
5. Seasons | 四季
Why do we have four seasons? The primary cause is the 23.5° tilt of Earth’s axis relative to its orbital plane, not the changing distance from the Sun.
为什么会有四季？主要原因是地轴相对于轨道平面倾斜23.5°，而不是地球到太阳距离的改变。
In June, the North Pole leans toward the Sun. The Northern Hemisphere receives more direct sunlight and longer days, producing summer. Meanwhile, the Southern Hemisphere has winter.
六月份，北极向太阳倾斜。北半球获得更直射的阳光和更长的白昼，形成夏季；与此同时，南半球处于冬季。
The Earth’s orbital distance actually varies by only about 3%. This small variation is far less important than the effect of axial tilt.
地球的轨道距离实际变化仅约3%。这个微小的变化远不如地轴倾斜产生的影响重要。
6. The Sun’s Declination | 太阳赤纬
The declination of the Sun is its angular height above the celestial equator. It changes from about +23.5° in June to −23.5° in December.
太阳赤纬是太阳在天赤道以上的角度。它从六月的约＋23.5°变化到十二月的约−23.5°。
At the equator, the Sun passes overhead twice a year. At the tropics, it passes overhead once a year at the solstices.
在赤道，太阳每年有两次经过天顶。在回归线上，太阳每年只在至日经过天顶一次。
This change in declination drives the yearly pattern of solar altitude in every location on Earth.
赤纬的变化驱动了地球上每个地点太阳高度的季节变化模式。
Solar declination range: −23.5° to +23.5°
7. The Length of Daylight | 昼长变化
During summer in the Northern Hemisphere, days are longer than 12 hours. During winter, days are shorter than 12 hours. At the equator, daylight remains almost exactly 12 hours all year.
北半球夏季，白昼长于12小时；冬季，白昼短于12小时。赤道附近全年白昼几乎都是12小时。
A simple model for IGCSE: the length of daylight depends on both latitude and solar declination. Higher latitudes experience more extreme differences between summer and winter.
IGCSE的简单模型：昼长取决于纬度和太阳赤纬。纬度越高，冬夏昼长的差异就越大。
Inside the Arctic Circle, there is at least one full day with no sunrise in winter and one full day with no sunset in summer.
在北极圈内，冬天至少有一天完全没有日出，夏天至少有一天完全没有日落。
8. Solstices and Equinoxes | 冬至夏至与春分秋分
The summer solstice in the Northern Hemisphere occurs around June 21. The Sun is directly above the Tropic of Cancer. The winter solstice occurs around December 21, when the Sun is above the Tropic of Capricorn.
北半球夏至大约在6月21日，太阳直射北回归线。冬至大约在12月21日，太阳直射南回归线。
An equinox occurs when the Sun is directly above the equator. Around March 21 and September 23, day and night are approximately equal in length everywhere.
春分和秋分时，太阳直射赤道。大约3月21日和9月23日，全球各地昼长约等于夜长。
These four points on the orbit divide the year into four roughly equal parts and define the beginning of astronomical seasons.
轨道上的这四个点把一年分成大致相等的四部分，也标志着天文季节的开始。
9. Time Zones and the 365-Day Clock | 时区与365天时钟
Because the Earth rotates once per day, the Sun appears to move across the sky. To coordinate time across the world, we divide the planet into 24 time zones, each about 15° of longitude wide.
因为地球每天自转一圈，太阳看起来在天空移动。为了协调全球时间，我们把地球分成24个时区，每个时区大约跨15°经度。
One year contains about 365 rotations, but each rotation is observed as a solar day. This close connection between rotation and daylight keeps our 24-hour clock tied to the 365-day cycle.
一年包含约365次“太阳自转”，每一次自转对应一个太阳日。这种自转与白昼之间的紧密联系，使得我们的24小时制时钟与365天周期保持一致。
Without leap years, the calendar would drift off by about one day every four years, and eventually seasons would shift to different months.
如果没有闰年，日历每四年就会偏差约一天，最终季节会错移到不同的月份。
10. Biological Rhythms and the Year | 生物节律与年周期
Many living organisms have annual rhythms called circannual cycles. These include migration, hibernation, and flowering. They are influenced by changes in day length and temperature.
许多生物具有以年为周期的节律，称为年节律。这包括迁徙、冬眠和开花。它们受昼长和温度变化的影响。
In plants, seasonal changes in light duration trigger flowering. This is known as photoperiodism. Some plants flower when nights are long, while others need short nights.
在植物中，光照时长的季节变化会触发开花，这被称为光周期现象。有些植物在长夜条件下开花，另一些则需要短夜。
In humans, the annual cycle is less obvious, but our sleep-wake rhythm, hormone levels, and vitamin D production can vary with seasons.
在人类中，年周期不太明显，但我们的睡眠-觉醒节律、激素水平和维生素D生成都可能随季节变化。
11. Why 365? Measuring Time | 为什么是365天？
The number 365 is not chosen for convenience; it is a fact of nature. It comes from the time the Earth takes to complete one orbit around the Sun relative to the seasons.
365这个数字并非为了方便而选择，而是自然事实。它来自地球绕太阳完成一圈与季节相对的平均时间。
Ancient calendars used lunar months, which made 12 months equal to about 354 days. That is why solar calendars were developed to keep festivals in their proper seasons.
古代日历使用朔望月，12个月约354天。这就是为什么人们发展出太阳历，以确保节日对应当中的季节。
Modern timekeeping uses atomic clocks, but even so, one International Astronomical Union year is defined as exactly 365.25 days of 86,400 seconds for astronomical calculations.
现代计时使用原子钟，即便如此，国际天文学联合会定义的一年在天文计算中仍等于365.25个86400秒。
1 year = 365.25 days × 24 h/day × 60 min/h × 60 s/min = 31,557,600 s
12. Conclusion | 总结
The number 365 encapsulates one full cycle of our planet around the Sun. It explains seasons, leap years, day length, and even biological rhythms.
数字365浓缩了地球绕太阳一整圈的运动。它解释了季节、闰年、昼长，甚至生物节律。
Understanding the science of the year helps us appreciate why our calendar works and how Earth’s motion shapes life on our planet.
理解一年的科学，有助于我们明白日历为什么这样运作，以及地球运动如何塑造我们星球上的生命。
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